The present invention relates to a parking brake device used in a vehicle such as a forklift.
For example, a forklift includes a parking brake device used when parking the forklift besides a service brake device used for controlling the speed or stopping the forklift during traveling. Such a parking brake device is disclosed in, for example, Toyota Jidoshokki Technical Review No. 46, January 2003, on page 18-21.
As shown in FIG. 4, the forklift is provided with a braking portion 101 of a parking brake device in the vicinity of a drive motor 102. The braking portion 101 includes a brake disk 103, which rotates integrally with an output shaft 102a of the drive motor 102, and a piston 104, which has a brake pad 104a. The brake pad 104a moves relative to the brake disk 103 in the axial direction of the output shaft 102a. The braking portion 101 also includes a spring 105, which urges the piston 104 in a direction such that the brake pad 104a approaches the brake disk 103, and a control oil chamber 106, which applies force on the piston 104 in a direction opposite to the force of the spring 105 based on the internal pressure. The control oil chamber 106 is connected to a hydraulic pump 107 and an oil tank 108 via a switching valve 109.
When an occupant manipulates the switching valve 109 such that the control oil chamber 106 is connected to the hydraulic pump 107, hydraulic oil is supplied to the control oil chamber 106 from the hydraulic pump 107. Therefore, the piston 104 moves against the force of the spring 105 reducing the contact force between the brake pad 104a and the brake disk 103. Accordingly, the parking brake device loses a braking force. When the occupant manipulates the switching valve 109 such that the control oil chamber 106 is connected to the oil tank 108 without via the hydraulic pump 107, hydraulic oil in the control oil chamber 106 is discharged to the oil tank 108. Therefore, the piston 104 moves by the force of the spring 105 increasing the contact force between the brake pad 104a and the brake disk 103. Accordingly, the parking brake device exerts a braking force.
Unlike the service brake device, which reflects the intention of the occupant to the level of the braking force, the parking brake device is switched between tow positions, which are a braking state and a non-braking state. Therefore, for example, if the parking brake device is shifted from the non-braking state to the braking state when the forklift is traveling, the maximum value of the absolute value of deceleration (so called “braking G-force”) might be increased.